If you want to keep track of which accounts have stopped following you on Instagram or identify which accounts didn’t follow you back use Follow Cop – an excellent free app for android phones.
Simply download the app, login using your Instagram logins and it will give the choice of
a) Seeing who didn’t follow you back
b) Who recently unfollowed you
The latter is very handy for identifying the accounts that use automated tools to follow you and then quietly unfollow you a week or so later which is very annoying. With this tool you can immediately unfollow them back once you spot it.
Download it here